1. Field of the Invention
This invention relates in general to electrophotography and, more specifically, to a flexible organophotoreceptor having a novel electrically conductive ground stripe.
2. Background of the Art
In electrophotography, an organophotoreceptor in the form of a plate, belt, or drum having an electrically insulating photoconductive element on an electrically conductive substrate is imaged by first uniformly electrostatically charging the surface of the photoconductive layer, and then exposing the charged surface to a pattern of light. The light exposure selectively dissipates the charge in the illuminated areas, thereby forming a pattern of charged and uncharged areas. A liquid or solid toner is then deposited in either the charged or uncharged areas to create a toned image on the surface of the photoconductive layer. The resulting visible toner image can be transferred to a suitable receiving surface such as sheets of material, including, for example, paper, polymeric film, metal, metal coated substrates, composites and the like. The imaging process can be repeated many times on the reusable photoconductive layer.
Both single layer and multilayer photoconductive elements have been used. In the single layer embodiment, a charge transport material and charge generating material are combined with a polymeric binder and the combined materials are then deposited on the electrically conductive substrate. In the multilayer embodiment, the charge transport material and charge generating material are in the form of separate layers, each of which can optionally be combined with a polymeric binder, and deposited on the electrically conductive substrate. At least two arrangements are possible. In one arrangement (the xe2x80x9cdual layerxe2x80x9d arrangement), the charge generating layer is deposited on the electrically conductive substrate and the charge transport layer is deposited on top of the charge generating layer. In an alternate arrangement (the xe2x80x9cinverted dual layerxe2x80x9d arrangement), the order of the charge transport layer and charge generating layer is reversed.
In both the single and multilayer photoconductive elements, the purpose of the charge generating material is to assist in the generation of charge carriers (i.e., holes or electrons) upon exposure to light. The purpose of the charge transport material is to assist in accepting these charge carriers and transport them through the charge transport layer in order to discharge a surface charge on the photoconductive element.
In order to properly image an organophotoreceptor, the electrically conductive substrate must be brought into electrical contact (e.g., by establishing an electrically conductive path) with a source of fixed potential elsewhere in the imaging device. This electrical contact must be effective over many thousands of imaging cycles in automatic imaging devices. Since the electrically conductive substrate frequently comprises a thin metal layer on a thermoplastic film, long life cannot be achieved easily with an ordinary electrical contact that rubs directly against the thin metal layer. One approach to minimize the wear of the thin metal layer is to use a grounding brush. However, the grounding brush still wears or abrades the thin metal layer over time.
Another approach to improving electrical contact between the electrically conductive substrate of flexible organophotoreceptors and a grounding means is the use of an electrically conductive ground stripe. The stripe is in contact with the electrically conductive substrate and is adjacent to one edge of the photoconductive element. Generally the ground stripe comprises opaque conductive particles dispersed in a film-forming binder. This approach to grounding the electrically conductive substrate increases the overall life of the organophotoreceptor, because the ground stripe is more durable than the thin metal layer. However, such a ground stripe is still subject to erosion and wetting problems.
Erosion is particularly severe in electrophotographic imaging systems utilizing metallic grounding brushes or sliding metal contacts. The conductive particles formed during erosion of the ground stripe tend to drift and settle on other components of the machine to adversely affect machine performance and appearance. Another problem due to the erosion of the ground stripe is that the electrical conductivity of the ground stripe can decline to unacceptable levels after extended usage.
Erosion of the ground stripe also causes problems in systems using a timing light in combination with a timing aperture or a timing reflective mark in the ground stripe for controlling various functions of imaging devices, such as seam detection, cycle detection, and registration. If a timing aperture is used and the ground stripe and the underlying thin metal layer are worn away by erosion, the eroded area becomes transparent and thereby allows light to pass through it. This can create false timing signals, which in turn can cause the imaging device to malfunction. If a timing reflective mark is used and the ground stripe is worn away by erosion and the thin metal layer is exposed, the eroded area becomes reflective and thereby reflects light and creates false timing signals which also cause the imaging device to malfunction.
A wetting problem arises when a ground stripe dispersion is coated on a sub-layer on the thin metal layer and the dispersion does not spread uniformly. Such a wetting problem results in pinholes in the ground stripe after drying. The pinholes cause similar timing problems as described in the previous paragraph.
The above-mentioned problems associated with the use of a ground stripe in flexible organophotoreceptors are undesirable in electrophotographic imaging systems. Therefore, there is a need to improve the durability and conductivity stability of the ground stripe, and the wettability of the ground stripe formulation.
One aspect of this invention is to provide a ground stripe which overcomes the above-mentioned disadvantages so that it provides an organophotoreceptor having a durable life, a stable conductivity over time, and no pinholes.
In a first aspect, the invention features an organophotoreceptor that includes
(a) at least one photoconductive element capable of retaining an electrophotographic latent image;
(b) an electrically conductive substrate having an electrically conductive surface; and
(c) an electrically conductive ground stripe adjacent to the photoconductive element and in electrical contact with the electrically conductive substrate, wherein the electrically conductive ground stripe comprises a film-forming binder, conductive particles, inorganic particles having a Moh hardness greater than 5 (it is possible to provide both the conductive function and the Mohs hardness function in a single particle, but different particles are preferred), and a surfactant;
(d) at least one charge transport compound; and
(e) at least one charge generating compound
In a second aspect, the invention features an electrophotographic imaging apparatus that includes (a) a plurality of support rollers, at least one support roller having a diameter no greater than about 40 mm; and (b) the above-described organophotoreceptor being in the form of a flexible belt threaded around the support rollers (e.g., a flexible support such as a belt having a flexible coating on at least one surface thereof, the coating comprising a binder, surfactant, conductive particles, and inorganic particles having a Moh hardness greater than 5). The apparatus preferably further includes a liquid toner dispenser.
In a third aspect, the invention features an electrophotographic imaging process that includes (a) applying an electrical charge to a surface of the above-described organophotoreceptor; (b) imagewise exposing the surface of the organophotoreceptor to radiation to dissipate charge in selected areas and thereby form a pattern of charged and uncharged areas on the surface; (c) contacting the surface with a liquid toner that includes a dispersion of colorant particles in an organic liquid to create a toned image; and (d) transferring the toned image to a substrate.
In a fourth aspect, the invention features an electrically conductive ground stripe that includes:
(a) a film forming binder;
(b) conductive particles;
(c) inorganic particles having a Mohs hardness greater than 5; and
(d) a surfactant.
In a fifth aspect, the invention features a process of making a ground stripe comprising the steps of:
(a) dispersing inorganic particles having a Mohs hardness greater than 5 in a first liquid to form an inorganic particle dispersion;
(b) dispersing a mixture of conductive particles, a surfactant, a binder, and the inorganic particle dispersion in a second liquid to form a ground stripe dispersion;
(c) coating the ground stripe dispersion on a substrate; and
(d) drying the ground stripe dispersion on the substrate.
It is to be noted that even though these steps are described in an apparent order, restriction to that order is not necessarily essential. For example, a first blend of mixture of conductive particles, a surfactant, and a binder may be dispersed, a dispersion made of the hard particles in the first liquid, and then the complete dispersion step (b) would be performed. Similarly, where the term dispersion is used, a suspension that is sufficiently stable as to enable the coating to be applied may also be used, although a true dispersion is preferred.
In a sixth aspect, the invention features a process of making a ground stripe comprising the steps of:
(a) dispersing inorganic particles having a Mohs hardness greater than 5 in a first liquid to form an inorganic particle dispersion;
(b) dispersing a mixture of conductive particles, a surfactant, and a binder in a second liquid to form a conductive particle dispersion;
(c) mixing or dispersing the inorganic particle dispersion and the conductive particle dispersion together to form a ground stripe dispersion;
(d) coating the ground stripe dispersion on a substrate; and
(e) drying the ground stripe dispersion on the substrate
In a seventh aspect, the invention features a process of making a ground stripe comprising the steps of:
(a) dispersing inorganic particles having a Mohs hardness greater than 5 in a first liquid to form an inorganic particle dispersion;
(b) dispersing a mixture of conductive particles, a binder, and the inorganic particle dispersion in a second liquid to form a ground stripe dispersion;
(c) mixing or dispersing the ground stripe dispersion with a surfactant;
(d) coating the ground stripe dispersion on a substrate; and
(e) drying the ground stripe dispersion on the substrate.
In addition to drying the coating, the binder component and/or part of the liquid component may be polymerizable or crosslinkable, so the xe2x80x98dryingxe2x80x99 step could include polymerization, curing, thermal polymerization, photopolymerization, and the like.
The invention provides novel ground stripes for organophotoreceptors featuring a combination of good mechanical, conductive, and electrostatic properties. These organophotoreceptors can be used successfully with powder or liquid toners to produce high quality images. The high quality of the images is maintained after repeated cycling.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.